Strainer assembly

ABSTRACT

A strainer system comprises a strainer housing having an inlet for raw fluid and an outlet for strained fluid; a generally cylindrical strainer basket having a strainer screen and disposed within the housing, the external surface of the strainer screen being in flow communication with the raw fluid inlet and the internal surface of the strainer screen being in flow communication with the strained fluid outlet; a backwash collector assembly comprising a backwash conduit having an opening disposed adjacent the outer surface of the strainer screen, the backwash conduit and the strainer basket being moveable relative to each other, so as to allow the opening to pass across the external surface of the strainer screen. A method of straining a fluid, in particular a liquid, is also provided. Finally, a valve assembly for use in the selective opening and closing of a plurality of backwash conduits is disclosed.

The present invention concerns a strainer assembly and its method ofoperation. The assembly and method are suitable for use in a wide rangeof separation operations, in particular, but not limited to, thestraining of sea water for use in offshore oil and gas exploration andproduction operations.

The removal of solid material from water and aqueous streams is requiredin many processes and operations. One such example is the straining ofsea water required, in order to remove solid material including mud,sand, gravel and plant and animal matter, enabling the water to be usedin drilling and other operations associated with the exploration andproduction of oil and gas. Other applications for straining include thecleaning of water produced from subterranean wells, including oil andgas wells, the cleaning and purification of aquifer water, river waterand estuary water.

Known strainer assemblies comprise one or more strainer screensextending across a fluid flow path in a vessel or conduit between aninlet for raw fluid and an outlet for strained fluid, the fluid beingcaused to pass through the screen. In a simple configuration, thestrainer screen extends across a pipe or vessel through which the fluidbeing cleaned is passed. In order to increase the capacity of anassembly, it is known to arrange the strainer material into one or morebaskets or candles. One or more such baskets may be arranged within avessel, to provide a higher surface area of strainer screen per unitvolume of the vessel or conduit.

As straining proceeds, solid material is retained on the screen. Theaccumulation of solid material causes an increase in the pressure dropacross the screen, in turn reducing the volume flowrate of fluid throughthe strainer, leading to reduced straining efficiency. Systems andprocedures for removing the solid build up on strainer screens are knownand generally comprise reversing the flow of fluid, such that a fluid,often a portion of the strained fluid, is passed through the screen inthe opposite direction, removing the solid material. Such a procedure isknown in the art as ‘backwash’. The strainer system generally comprisesa separate conduit and outlet for removing the backwash fluid and theentrained solid debris. In the simple strainer configuration mentionedhereinbefore, the backwash assembly is similarly simple andstraightforward. However, when more complex strainer assemblies areemployed, the backwash system increases in complexity. This isparticularly the case when it is desired to conduct a backwash cycle ona portion of the strainer assembly, while still keeping the remainder ofthe system available for use.

In one known arrangement, each strainer basket or candle is located inits own conduit, having a fluid inlet and a fluid outlet. Several suchconduits are connected by their respective inlets and outlets to aseries of fluid headers. In normal operation, raw fluid is feed via afeed header to each strainer candle. Strained fluid is removed via anoutlet header. By the operation of appropriate valves, each strainercandle may be isolated from the feed and outlet headers and connectedinstead to a backwash inlet header and drain header, by means of which abackwash fluid may be passed in reverse direction through the strainercandle to remove solid material and clean the strainer screen. It willbe appreciated that such an arrangement is complex in terms of thepipework and valve arrangements required. In addition, the overallassembly occupies a relatively large volume; a disadvantage when theavailable space is at a premium, for example on a subsea wellheadinstallation or offshore production platform.

In an alternative known arrangement, a plurality of individual,generally cylindrical strainer candles are mounted in a single vessel.In one particular assembly, the strainer candles extend longitudinallywithin the vessel between two supporting plates, each plate having anopening in communication with the interior of each strainer candle. Aninlet for raw fluid is provided in one end of the vessel, whereby theraw fluid is caused to flow through the respective support plate andinto the bore of each candle. Strained fluid is collected from aroundthe candles and leaves the vessel through an appropriate outlet. Solidparticles in the raw fluid are retained on the inner surface of thestrainer candles.

A backwash system is provided to clean the strainer candles and removethe accumulation of solid material. In its simplest form, the backwashsystem simply reverses the direction of fluid flow through the vesseland the strainer candles. However, in order for the backwash operationto be completed, the entire vessel must be taken off line.

An improved backwash assembly applies the backwash procedure to one orseveral strainer candles, while leaving the remaining candles inoperation. The system comprises a back wash assembly having one or morerotatable backwash conduits in one end of the vessel. The back washconduits are brought into cooperation with successive strainer candlesand cause fluid to flow along the interior of the candle, removing theaccumulated solids by means of a shearing fluid flow. This system iscomplex to construct and maintain. It relies upon a seal being formedbetween the backwash conduit and the end of the strainer candle and isprone to leaks and poor backwash efficiency when the seals fail. Animproved strainer and backwash system is therefore required.

One such system is the METROL® SEA-SCREEN® Coarse Strainer, commerciallyavailable from Petreco International. The system comprises one or morecylindrical strainer baskets arranged within a generally cylindricalvessel. Each strainer basket is provided with a dedicated backwashassembly having an elongate backwash collector head extending the lengthof the respective strainer basket. The collector head comprises a slotor a plurality of slots adjacent the inner surface of the strainerbasket and connected to a backwash conduit in the backwash assembly. Inoperation, the collector head is rotated within the strainer basket soas to cause the slot to pass across the inner surface of the basket. Thebackwash conduit is connected to an outlet at reduced pressure,typically by being vented to atmospheric pressure, causing fluid withinthe vessel to enter the collector head through the slot. Fluid is drawnboth through the strainer basket in the reverse direction to normal flowin the region adjacent the collector head, as well as across the innersurface of the strainer basket. In this way, solids accumulated on theinner surface of the strainer basket are removed by a reverse and ashear fluid flow.

While the aforementioned system provides a high straining capabilitywith a very high backwash efficiency, there is a need for still furtherimprovements for some specific applications of the straining concepts.For example, the system requires a motor and gearbox assembly for eachstrainer basket. While this is generally acceptable, there arecircumstances where the overall weight of the strainer unit needs to bereduced. It would also be an advantage if the surface area of strainingmaterial per unit volume of the containment vessel could also beincreased. This would allow the diameter of the vessel to be decreasedfor a given straining duty, reducing the size of the unit and furtherreducing the overall weight of the vessel, both empty and when filledwith fluid and in operation.

Surprisingly, it has been found that a most advantageous strainer systemcan be provided by reversing the direction of fluid flow that iscommonly applied in the art and employing the backwash collectorarrangement of the METROL® SEA-SCREEN® Coarse Strainer on the exteriorof the strainer basket.

Accordingly, in a first aspect, the present invention provides astrainer assembly comprising a strainer housing having an inlet for rawfluid and an outlet for strained fluid;

a strainer basket having a strainer screen and disposed within thehousing, the external surface of the strainer screen being in flowcommunication with the raw fluid inlet and the internal surface of thestrainer screen being in flow communication with the strained fluidoutlet;a backwash collector assembly comprising a backwash conduit having anopening disposed adjacent the outer surface of the strainer screen, thebackwash conduit and the strainer basket being moveable relative to eachother, so as to allow the opening to pass across the external surface ofthe strainer screen.

The strainer assembly of the present invention provides the significantadvantage that a high surface area of strainer screen can be containedwithin a given unit volume of housing, thus reducing the overall sizeand weight of the complete assembly. Further, the arrangement of thepresent invention provides for a very high backwash flowrate to beachieved, thus increasing the efficiency of the backwash operation. Thisin turn reduces the time taken to clean the strainer screen, maximizingthe time available for the straining of the liquid.

The strainer assembly of the present invention may be used to strain anysuitable liquid. However, the assembly is particularly suitable for thestraining of water and aqueous streams. In one specific application, theassembly of the present invention may advantageously be employed in thestraining of sea water, in particular sea water for injection into anunderground formation during an oil and/or gas production operation.

As noted above, the backwash conduit and the strainer basket are movablewith respect to one another. The backwash conduit may be arranged to bemovable, with the strainer basket being fixed. However, in a preferredembodiment, the strainer basket is movable, in particular rotatable,with respect to the backwash conduit, whereby successive portions of theouter surface of the strainer screen may be exposed to the opening inthe backwash conduit.

The backwash conduit may take any suitable form, providing a channel orconduit for the transport of the backwash liquid and the entrainedsolids removed from the strainer screen out of the strainer assembly fortreatment and/or disposal. Preferably, the backwash conduit is a tube,the opening in the backwash conduit being one or more slits or holes inthe tube. The slits or holes may take any suitable form. However, it ispreferred that the backwash conduit comprises one or more longitudinalslits. In a particularly advantageous arrangement, the opening in thebackwash conduit is a longitudinal slit extending substantially thecomplete length of the strainer screen.

The strainer assembly of the present invention may comprise a singlestrainer basket and a single backwash assembly. Alternatively, aplurality of pairs of strainer baskets and backwash conduits may behoused within a single vessel. However, it is an advantage of thepresent invention that a single backwash conduit can service a pluralityof strainer baskets, such that a plurality of strainer baskets isassociated with the backwash conduit. In such an arrangement, thebackwash conduit preferably comprises an opening for each of theplurality of strainer baskets.

The plurality of strainer baskets may be arranged in any suitablepattern around the backwash conduit. A most effective arrangement is forthe plurality of strainer baskets to be arranged radially around thebackwash conduit. In this way, the minimum amount of space is occupiedby the assembly. As will be discussed in more detail hereinafter, thisarrangement also allows a particularly effective drive assembly to beemployed to move the backwash conduit and strainer baskets relative toone another.

The number of strainer baskets arranged around the backwash conduit mayvary, depending upon the size and configuration of the strainer baskets,the backwash conduit, the housing, and the duty to be performed. Inparticular, the number of strainer baskets that may be arranged around asingle backwash conduit may range from 2 to 10. It has been found thatan arrangement comprising from 2 to 5 strainer baskets is particularlyadvantageous, with 3 strainer baskets arranged radially around onebackwash conduit being a preferred arrangement.

The arrangement of having a plurality of strainer baskets arrangedaround a single backwash conduit allows for the present invention to beapplied in a modular approach. Accordingly, a strainer system may beenvisaged comprising a plurality of strainer modules, each modulecomprising a backwash conduit and at least one strainer basketassociated with the backwash conduit. A single vessel may house 2 ormore such modules. Preferably, each module comprises a plurality ofstrainer baskets associated with each backwash conduit.

A further advantage of the strainer modules of the present invention isthat a single drive system can be arranged for moving each of thebackwash conduits with respect to its associated strainer baskets. Inparticular, a preferred assembly comprises a single drive system formoving all of the strainer baskets relative to their associated backwashconduit.

In addition, the present invention allows for a simplified system ofvalves in the backwash assembly, in order to perform the backwashoperation on each strainer basket. In particular, in the modulararrangement discussed hereinbefore, the assembly may comprise a valveassembly for selectively opening each of the backwash conduits toinitiate a backwash operation for the strainer baskets associated withthat backwash conduit. Details of a preferred valve assembly are givenhereinafter.

The opening in the backwash assembly may be arranged to be very close tothe outer surface of the strainer screen of the respective strainerbasket. In this way, during the backwash cycle, liquid is drawn in thereverse direction through the strainer screen, thereby removing thesolids trapped on the outer surface. However, in a preferredarrangement, the opening in the backwash conduit is disposed at adistance from the external surface of the strainer basket that, during abackwash operation, liquid is drawn both through the strainer screenfrom within the strainer basket and from outside the strainer basketacross the external surface of the strainer screen. This combination ofboth through-flow and cross-flow liquid provides a higher cleaningefficiency during the backwash cycle, improving the straining capacityof the assembly and shortening the backwash time. The opening in thebackwash conduit is preferably disposed such that, during a backwashoperation, the ratio of the liquid flow across the strainer screen tothe liquid flow through the strainer screen is from 3:1 to 1:3, morepreferably 2:1. To achieve this, the opening in the backwash conduit ispreferably at a distance of from 0.5 to 10.0 mm from the externalsurface of the strainer screen, more particularly from 0.75 to 3.0 mmfrom the external surface of the strainer screen, depending upon thesize of the solid particles in the inlet liquid stream.

The size of the opening in the backwash conduit may be selected toprovide the required backwash performance, which will depend in partupon the duty being performed. Preferably, the area of the opening inthe backwash conduit is from 0.1 to 10% of the surface area of thestrainer screen, more preferably from 0.1 to 4% of the surface area ofthe strainer screen.

It is preferred that the backwash conduit obscures only a minor portionof the strainer screen, thus allowing the major portion of the screen tooperate fully in the straining of liquid. Preferably, the backwashconduit obscures less than 10% of the exterior surface of the strainerscreen.

The strainer assembly may be arranged such that the straining functionis stopped for a particular strainer basket while the backwash operationis being conducted. Preferably, however, the backwash conduit isarranged such that, during the backwash operation, the portion of thestrainer screen away from the opening in the backwash conduit remainsoperative in the straining operation. The ability to carry out thebackwash and straining operations simultaneously using a given strainerbasket is a particular advantage of the present invention.

In a further aspect, the present invention provides a method ofstraining a liquid comprising:

causing the liquid to pass from the outside of a strainer basket througha strainer screen to the inside of the strainer basket;

causing a backwash conduit having an opening disposed adjacent the outersurface of the strainer screen to move relative to the outer surface ofthe strainer screen, whereby the portion of the strainer screen adjacentthe opening in the backwash conduit is subjected to a backwashoperation.

As noted above, the straining operation and the backwash operation maybe carried out successively on a given strainer basket. However,preferably, the straining and backwash operations are continuedconcurrently for a given strainer basket.

The backwash operation preferably comprises liquid flowing from insidethe strainer basket through the strainer screen and into the opening inthe backwash conduit and liquid flowing from adjacent the outside of thestrainer basket across the strainer screen, as discussed above.Preferably, the ratio of the flow of liquid through the strainer screenand into the opening in the backwash conduit and the flow of liquidacross the strainer screen into the opening in the backwash conduit isfrom 3:1 to 1:3, more preferably 1:2.

The backwash conduit may be moved relative to the strainer screen of thestrainer basket, which may also move or be stationary. However, it ispreferred that the backwash conduit is stationary and the strainerscreen is moved relative to the opening in the backwash conduit.

Most advantageously, a plurality of strainer baskets are associated withthe backwash conduit. Preferably, the backwash conduit comprises anopening for each of the plurality of strainer baskets.

The area of the opening in the backwash conduit is from 0.1 to 10% ofthe surface area of the strainer screen, more preferably from 0.1 to 4%of the surface area of the strainer screen.

As noted above, the concept underlying the present invention allows fora plurality of separate strainer baskets to be arranged and associatedwith a single backwash conduit. Accordingly, in a further aspect, thepresent invention provides a strainer system comprising:

a strainer housing having an inlet for raw fluid and an outlet forstrained fluid;

a plurality of strainer baskets disposed within the housing, one of theinternal and external surfaces of the strainer basket being in flowcommunication with the raw fluid inlet and the other of the internal andexternal surfaces of the strainer basket being in flow communicationwith the strained fluid outlet;a backwash collector assembly comprising a backwash conduit operable toconduct a backwash operation on the plurality of strainer baskets.

Most advantageously, the backwash collector assembly is arranged toconduct a backwash operation on the plurality of strainer basketssimultaneously.

In still a further aspect, the present invention provides a method ofstraining a fluid comprising:

causing the fluid to pass from the outside of each of a plurality ofstrainer baskets through a strainer screen to the inside of eachstrainer basket;

causing a backwash conduit having an opening to move relative to theouter surface of each strainer screen, whereby the portion of thestrainer screen adjacent the opening in the backwash conduit issubjected to a backwash operation.

The plurality of strainer baskets are preferably subjected to a backwashoperation simultaneously.

Finally, as noted above, the present invention advantageously allows aplurality of separate strainer modules, each comprising one or morestrainer baskets, to be serviced by a single drive system, in particulara single drive motor, and a single backwash system. In this respect, thepresent invention also provides, in a further aspect, a valve assemblyfor use in the backwash assembly of a strainer or filtration assembly,the backwash assembly comprising a plurality of backwash conduits eachhaving an outlet opening, the valve assembly comprising a valve platehaving an opening therein, the valve plate being moveable to bring theopening into alignment with the outlet opening of a backwash conduit tothereby connect the backwash conduit with an outlet conduit.

The valve plate is preferably rotatable, rotation of the valve platecausing the opening therein to move into and out of alignment withsuccessive backwash conduits. In a preferred arrangement, the opening isan arcuate opening. This allows the opening to be formed in the valveplate such that the length of the arc of the opening determines thelength of time that the valve is open. For operation of the strainerassembly of the present invention, the opening preferably extendsthrough an arc of from 10 to 30°, more preferably from 15 to 25°. Theopening may be sized so that a single backwash conduit is opened at anytime. Alternatively, the opening may be sized to allow two or morebackwash conduits to be opened.

While the valve may be arranged such that there are periods when nobackwash conduits are open, the resulting hydraulic clamping force maymake rotation of the valve plate difficult. This is particularly thecase where the motor driving the valve plate has insufficient torque toovercome the hydraulic clamping force. In such cases, it is preferredthat the valve plate and opening are arranged so that there is always atleast one backwash conduit at least partially open at any given time orposition in operation.

While the valve assembly of the present invention has been disclosed inconjunction with the backwash system of a strainer or filtrationassembly, it will be understood that the principles of the valveassembly may be applied to the opening and closing of any range ofopenings, tubes or conduits, as need may dictate.

Embodiments of the present invention will now be described, by way ofexample only, having reference to the accompanying drawings, in which:

FIG. 1 is a longitudinal cross-sectional view through a strainerassembly according to one embodiment of the present invention;

FIG. 2 is a cross-sectional view of the assembly of FIG. 1 along theline II-II;

FIG. 3 is a simplified representation of the arrangement of strainerbaskets and backwash conduit according to a preferred embodiment of thepresent invention; and

FIG. 4 is a cross-sectional view of the assembly of FIG. 1 along theline IV-IV showing the valve assembly of the present invention.

Referring to FIG. 1, there is shown a strainer assembly, generallyindicated as 2, comprising a housing 4 in the form of a generallyupright, cylindrical vessel of conventional design and construction.While the embodiment shown in the figures and discussed hereinafter isarranged essentially vertically, it will be appreciated that theassembly may be oriented in other positions, for example horizontally,as need dictates.

The housing has an upper domed end 6 secured to the central body 8 ofthe housing 4 by way of flanges 10 a and 10 b. The assembly 2 issupported on a base 12, allowing the assembly to be secured at anysuitable location. The housing 4 is provided with an inlet 14 for liquidto be strained and an outlet 16 for strained liquid.

A strainer assembly, generally indicated as 20, is retained within thehousing 4 between and upper plate 22 and a lower plate 24. The upperplate 22 is retained by being sandwiched between the flanges 10 a and 10b on the end 6 and body 8 of the housing 4. Suitable gaskets (not shown)are used to ensure a liquid-tight seal between the components and theflanges. As shown more clearly in FIG. 2, the strainer assembly 20comprises a plurality of strainer modules 26, each strainer modulecomprising a central backwash tube 28 and three strainer baskets 30. Asshown in FIG. 2, the arrangement shown in the figures comprises a totalof seven strainer modules 26. A single backwash tube 28 and a singlestrainer basket 30 are shown in FIG. 1, purely for reasons of clarity.

Each strainer module 26 is arranged substantially vertically within body8 of the housing 4. Each backwash tube 28 extends upwards from the lowerplate 24, the lower end of the backwash tube 28 being closed by thelower plate. At its upper end, the backwash tube 28 extends through theupper plate 22 and opens in the cavity in the upper end 6 of the housing4 above the upper plate.

Each backwash tube 28 is provided with a longitudinal slit 32 extendingalong its length at the point facing each of the strainer baskets 30associated with the backwash tube. This arrangement is shown in stylizedform in FIG. 4. As an alternative to the slit 32, the backwash tube maybe provided with a series of holes. A flow path for backwash liquid isthus formed extending from the region of the housing 4 surrounding thestrainer baskets 30, through the slits 32 in each backwash tube 28 andinto the upper cavity of the housing 4. An outlet (not shown for reasonsof clarity) for the backwash liquid is provided in the upper portion ofthe housing 4.

Each strainer basket 30 extends between the upper plate 22 and the lowerplate 24. At its upper end, each strainer basket 30 is closed and sealedat its upper end adjacent the upper plate 22. At its lower end, eachstrainer basket 30 extends through and opens into the cavity below thelower plate 24. Each strainer basket 30 comprises a generallycylindrical strainer screen 34 extending substantially the entire lengthof the strainer basket 30 between the upper plate 22 and the lower plate24. A straining flow path for liquid is thus formed extending from theinlet 14 and the cavity within the central portion 8 of the housing 4,through the strainer screen 34, downwards along the interior of thestrainer basket 30 and into the lower cavity in the housing 4 below thelower plate 24. The outlet 16 is provided to remove strained liquid fromthe housing 4.

In the arrangement shown in the figures, each backwash tube 28 and itsassociated strainer baskets 30 are movable with respect to one anotherby having the strainer baskets 30 rotatable. The drive assembly forrotating the strainer baskets 30 is shown in FIG. 1 and in detail inFIG. 2. A central drive gear 40 is provided, mounted on a central driveshaft 42 extending through the upper plate 22 and driven by a motorassembly (not shown for clarity). It is a particular advantage of theembodiment shown in the figures that all the strainer baskets are drivenfrom a single motor assembly.

Each strainer basket 30 is provided with a fixed gear 44 at its upperend adjacent the upper plate 22. The fixed gear 44 on each strainerbasket 30 is fixed in its relationship with the strainer basket, suchthat rotation of the gear 44 causes the entire strainer basket torotate. Each backwash tube 28 is provided with an idler gear 46 at itsupper end adjacent the upper plate 22. The idler gear 46 is free torotate about the backwash tube 28, without causing rotation of thebackwash tube. The idler gear 46 of each backwash tube 28 engages withthe fixed gear 44 on each of the associated strainer baskets 30. Theinnermost strainer basket 30, that is the basket disposed closest to thecentral axis of the housing 4, of each module has its fixed gear 44engaged with the central drive gear 40.

In operation, rotation of the central drive gear 40 causes rotation ofthe innermost strainer basket 28 of each module, which in turn drive theremaining strainer baskets by means of the idler gear 46 on eachbackwash tube 28.

In order to operate the backwash function to clean the strainer screenon each strainer basket, the assembly 2 is provided with a valveassembly. The valve assembly comprises a circular valve plate 50, shownin detail in FIG. 4. The valve plate 50 is mounted for rotation on theupper surface of the upper plate 22, as shown in FIG. 1. A backwashdrive assembly (not shown for clarity) is provided to rotate the valveplate 50. The valve plate 50 is provided with an elongate arcuateopening 52, positioned such that rotation of the valve plate 50 causesthe opening 52 to pass over the open end of each backwash tube 28 inturn. The length of the arcuate opening 52 determines the length of timethat each backwash tube 28 is opened, which in turn determines thelength of the backwash operation applied to each strainer module. Withthe width in the radial direction of the arcuate opening set as the sameas the internal diameter of each backwash tube, the length of time thateach backwash tube is open is proportional to the arc of the opening,for a given speed of rotation of the valve plate. This is preferablyfrom 10 to 30°. As shown in FIG. 4, the opening extends through an arcof 19°.

In order to provide support and rigidity to the internal structure ofthe strainer assembly, tie bars 60 extend within the housing 4 betweenand are bolted to the upper plate 22 and the lower plate 24. The tiebars are shown in FIG. 1, but have been omitted from the remainingfigures for reasons of clarity.

In operation, a liquid to be strained, such as seawater, is fed to theinlet 14 of the housing 4, from where it enters the central cavity ofthe assembly. The liquid passes through the strainer screen 34 of eachstrainer basket 30, the action of which is to leave any entrained solidson the outer surface of the strainer screen 34. The strained liquidflows along the strainer basket 30 and into the cavity below the lowerplate 24, from where it leaves the assembly through the outlet 16.

After a period of operation, the differential pressure across thestrainer screens will increase, due to the build up of solid materialand debris. At a predetermined pressure drop, the backwash operation isinitialized.

The backwash operation is commenced by starting the drive assembly torotate the strainer baskets 30 by means of the central drive gear 40.The action of this is to cause the entire outer surface of each strainerscreen 34 to pass across the respective slit or hole 32 in theassociated backwash tube 28. At the same time, the valve plate 50 isrotated, causing the arcuate opening 52 to pass into and out ofalignment with the upper end of each backwash tube 28 in turn. As theopening 52 passes over a backwash tube 28, the interior of the tube isvented to the pressure in the domed end 6 of the housing, which istypically ambient or atmospheric pressure. This establishes asignificant pressure differential between the liquid in the centralcavity of the housing and the outlet of the opened backwash tube 28, theeffect of which is twofold. First, liquid is drawn through the strainerscreen 34 in the reverse direction of normal flow, that is from insidethe strainer basket 30 to the outside and into the slit 32 in thebackwash tube 28. Second, liquid surrounding the strainer screen iscaused to flow across the outer surface of the strainer screen and intothe slit 32 in the backwash tube. Thus, a dual-flow backwash system isestablished.

The portion of the strainer screen 34 that is exposed to the slit in thebackwash tube 28 and the backwash flow is just a fraction of the totalsurface are of the strainer screen. Typically, this is from 0.1 to 10%of the total surface area. In this way, a very high rate of backwashflow is generated, leading to a very high cleaning efficiency of thescreen. Typically, backwash flow velocities of 2 m/s and higher can beachieved and maintained.

As the opening 52 in the valve plate 50 moves out of alignment with thefirst backwash tube 28, the backwash operation for that strainer modulewill cease. The next strainer module in the direction of rotation of thevalve plate 50 will begin its backwash cycle once the opening 52 in thevalve plate 50 comes into alignment with the backwash tube 28. In thearrangement shown in the accompanying figures, the valve plate 50 isprovided with a single opening 52 of such a size that only one backwashtube 28 can be open at any given time. In this way, only one moduleundergoes backwash at a time. If desired, more than one opening 52 maybe provided in the valve plate 50 or the opening may be larger, suchthat more than one strainer module may undergo backwash at the sametime.

It is a feature of the strainer assembly of this invention thatstraining continues while individual strainer modules are undergoing abackwash. Indeed, a given strainer basket may be having a portion of itsstrainer screen being cleaned by the backwash cycle discussed above,leaving the remaining area of the screen available for straining.

The length of time of operation of the backwash system will bedetermined by the operating circumstances, in particular theconcentration of solid material in the liquid being strained. Once thepressure differential across the strainer screens is reduced to normaloperating levels, the backwash operation is stopped and normal operationis continued. In such a case, it is envisaged that the backwash systemwill operate intermittently. However, the strainer assembly of thepresent invention is flexible in its operation, such that heavilypolluted liquids containing a high solids content may be strained andthe backwash system operated continuously.

The method and apparatus of the present invention, in their variousaspects, have been disclosed and described with reference to thestraining of fluids, in particular liquids. In this respect, strainingis to be understood as a surface effect, providing nominal separationbetween solids and the fluids in which they are entrained, for example98% removal of all particles greater than 100 μm. It will be understoodthat the various aspects of the invention may also be applied in otherfluid-solid separation processes, such as filtration, including depthfiltration and the like.

The present invention has been particularly described in relation to theseparation of solids from a liquid. However, the principles of thepresent invention in its various aspects may also be applied to theremoval of solids from fluids in general, including gaseous streams.

The invention claimed is:
 1. A strainer system comprising: a strainerhousing having an inlet for raw fluid and an outlet for strained fluid;a strainer basket having a strainer screen and disposed within thehousing, an external surface of the strainer screen being in flowcommunication with the raw fluid inlet and an internal surface of thestrainer screen being in flow communication with the strained fluidoutlet; a backwash collector assembly comprising a cylindrical backwashconduit having an opening disposed adjacent the external surface of thestrainer screen, the cylindrical backwash conduit and the strainerbasket being moveable relative to each other, so as to allow the openingto pass across the external surface of the strainer screen; a pluralityof strainer modules, each module comprising a cylindrical backwashconduit and at least one strainer basket associated with the cylindricalbackwash conduit; and a valve assembly for selectively opening each ofthe cylindrical backwash conduits to initiate a backwash operation forthe strainer baskets associated with that cylindrical backwash conduit,wherein the valve assembly comprises a valve plate having a single valveplate opening therein, the valve plate being moveable to bring thesingle valve plate opening into alignment with the outlet opening of acylindrical backwash conduit to thereby directly connect the cylindricalbackwash conduit with a single outlet conduit.
 2. The strainer systemaccording to claim 1, wherein the strainer basket is rotatable withrespect to the cylindrical backwash conduit, whereby successive portionsof the external surface of the strainer screen may be exposed to theopening in the cylindrical backwash conduit.
 3. The strainer systemaccording to claim 1, wherein the cylindrical backwash conduit is atube, the opening in the cylindrical backwash conduit being one or moreslits or holes in the tube.
 4. The strainer system according to claim 3,wherein the opening in the cylindrical backwash conduit comprises one ormore longitudinal slits or holes.
 5. The strainer system according toclaim 4, wherein the opening in the cylindrical backwash conduit is alongitudinal slit or hole extending substantially the complete length ofthe strainer screen.
 6. The strainer system according to claim 1,wherein a plurality of strainer baskets is associated with thecylindrical backwash conduit.
 7. The strainer system according to claim6, wherein the cylindrical backwash conduit comprises an opening foreach of the plurality of strainer baskets.
 8. The strainer systemaccording to claim 6, wherein the plurality of strainer baskets isarranged radially around the cylindrical backwash conduit.
 9. Thestrainer system according to claim 1, further comprising a single drivesystem for moving all of the strainer baskets relative to its associatedcylindrical backwash conduit.
 10. The strainer system according to claim1, wherein the opening in the cylindrical backwash conduit is disposedat a distance from the external surface of the strainer basket that,during a backwash operation, fluid is drawn both through the strainerscreen from within the strainer basket and from outside the strainerbasket across the external surface of the strainer screen.
 11. Thestrainer system according to claim 1, wherein the opening in thecylindrical backwash conduit is at a distance of from 0.5 to 10.0 mmfrom the external surface of the strainer screen.
 12. The strainersystem according to claim 1, wherein the area of the opening in thecylindrical backwash conduit is from 1 to 10% of the surface area of thestrainer screen.
 13. The strainer system according to claim 1, whereinthe cylindrical backwash conduit is arranged such that, during thebackwash operation, the cylindrical backwash conduit does not interferewith a portion of the strainer screen away from the opening in thecylindrical backwash conduit which remains operative in the strainingoperation.
 14. The strainer system according to claim 1, wherein thevalve plate is rotatable, rotation of the valve plate causing the singlevalve plate opening therein to move into and out of alignment withsuccessive cylindrical backwash conduits.
 15. The strainer systemaccording to claim 1, wherein the single valve plate opening is anarcuate opening.
 16. The strainer system according to claim 15, whereinthe single valve plate opening is formed in the valve plate such thatthe length of the arc of the single valve plate opening determines thelength of time that the valve is open.
 17. The strainer system accordingto claim 16, wherein the single valve plate opening extends through anarc of from 10 to 30°.
 18. A strainer system comprising: a strainerhousing having an inlet for raw fluid and an outlet for strained fluid;a plurality of strainer baskets disposed within the housing, one of aninternal surface and an external surface of the strainer basket being inflow communication with the raw fluid inlet and the other of theinternal and external surfaces of the strainer basket being in flowcommunication with the strained fluid outlet; a backwash collectorassembly comprising a cylindrical backwash conduit having an openingdisposed adjacent the external surface of the strainer screen, thecylindrical backwash conduit and the strainer basket being moveablerelative to each other, so as to allow the opening to pass across theexternal surface of the strainer screen, wherein the opening in thecylindrical backwash conduit is disposed at a distance of from 0.5 to 10mm from the external surface of the strainer basket so that, during abackwash operation fluid is drawn both through the strainer screen fromwithin the strainer basket and from outside the strainer basket acrossthe external surface of the strainer screen; and a valve assembly foropening the cylindrical backwash conduit to initiate a backwashoperation for the strainer baskets associated with that cylindricalbackwash conduit wherein the valve assembly comprises a valve platehaving a single valve plate opening therein, the valve plate beingmoveable to bring the single valve plate opening into alignment with theoutlet opening of the cylindrical backwash conduit to thereby directlyconnect the cylindrical backwash conduit with a single outlet conduit.19. The strainer assembly according to claim 18, wherein the cylindricalbackwash collector assembly is arranged to conduct a backwash operationon the plurality of strainer baskets simultaneously.